Pig-Mounted Experimental Measurement of In-Situ Absolute Biaxial Stress in Pipelines

This paper describes recent experimental work on the measurement of absolute levels of biaxial stress in ferromagnetic pipeline materials. A prototype in-line inspection tool has been constructed and has subsequently undergone evaluation in a series of pigging trials to determine its accuracy in recording of stress values. Recently attention has been raised by a number of groups working on the development of techniques intended to utilize the dependence of the magnetic response of ferrous materials to applied stress in order to directly measure stress. Most of these techniques have been based on the Barkhausen effect. This paper discusses the use of an alternative technique based on other magnetic properties that have been shown to allow derivation of a quantifiable relation between the level of stress present in material and the magnetic response. This technique, named MAPS by its developers, MAPS Technology, has been employed with considerable success out-with the pipeline industry. An earlier paper described the technique used to make the measurements. The response of pipe-wall materials to an applied alternating magnetic field is measured, and knowledge of the variation of magnetic properties of the material used to determine stress values. In order to prove the suitability of this technique to pipeline applications, a prototype inspection pig has been designed and constructed, and a series of tests performed in a 24″ test facility, constructed specifically for the purpose. The test facility design allowed for two modes of operation. The first of these allowed pigging through the 50m of test pipe, with an arrangement that allowed bending stresses to be applied to the pipe. The second mode of operation enabled internal pressure to be applied as the tool was pulled through the test section. The pig based system successfully gathered stress measurements in both axial and hoop directions. The pig measurements were subsequently compared to theoretical values of stress and to measurements taken with strain gauges and other instrumentation fitted to the test facility.